The general reaction of an aromatic amine with phosgene to obtain the corresponding aromatic isocyanate was first reported in 1844 by Hentschel in Berichte, 17, 1284. Various publications and patents have improved this process over the years since this first report.
The addition of a halogen molecule to the amine aromatic ring has the effect of deactivating the amine in its conversion with phosgene to the corresponding isocyanate. Because of this, the original phosgene technology had to be modified to synthesize polyhalogenated aromatic isocyanates.
The patent to Thompson No. 2,689,861 relates to the preparation of trihalogenated phenyl isocyanates by reacting the corresponding aromatic amine with phosgene in the presence of a minor amount of tetramethylurea However, reinvestigation of the process described in this patent reveals that it is not an effective procedure for preparing trihalogenated phenyl isocyanates because the tetramethylurea used as a weak organic base, promotes the formation of urea byproduct derivatives, thus reducing the yield of tri-halogenated phenyl isocyanate. Furthermore, the process described in the above-mentioned patent is not suitable in the preparation of tetra and pentahalogenated phenyl isocyanates.
The patent to Lichty et al. No. 2,362,648 relates to the preparation of isocyanates by reacting the corresponding amine with phosgene in the presence of a small amount of tertiary amine as catalyst. However the patent fails to disclose the method as feasible in preparing polyhalogenated phenyl isocyanates.